Why is my RV water pump pulsing when a tap is open?

Answered by RV Parts Giant · Upland, CA · September 26, 2026

Short answer: The pump is switching on and off rapidly because flow is so low that line pressure hits the cut-off almost as soon as it starts. Something downstream is restricting flow: a clogged aerator, a spent inline filter or a partly closed valve, or there is no accumulator to give the pump something to push against. Pull the tap aerator first; if flow improves, clean or replace it and the pulsing stops.

An RV water pump is a diaphragm pump with a pressure switch. It runs until line pressure reaches the cut-off, stops, and holds pressure against a check valve until you open a tap. Pulsing or hammering while a tap is open means the pump keeps reaching cut-off almost instantly: flow out of the tap is so low that pressure builds faster than the tap can bleed it. It is annoying rather than damaging, but it shortens switch life and makes a shower unpleasant, and it is usually a ten-minute fix.

Tell it apart from the other two symptoms

Runs briefly every few minutes with every tap closed: pressure is leaking away somewhere between the pump and the faucets. That is a leak hunt, not this fault.
Runs continuously and never shuts off: the pump cannot reach cut-off. Empty tank, air leak on the suction side, clogged strainer or a failed diaphragm.
Pulses or hammers while a tap is open: a restriction downstream. Read on.

The checks, in order

1. The aerator. Unscrew the aerator from the tap that pulses. If flow improves immediately, clean the screen or replace it. A clogged aerator is the most common cause on campers that fill from anything other than a treated supply.

2. The inline filter. A spent water filter restricts the whole system, and it is the usual culprit on campers that fill from creeks or tanks. Replace the cartridge and see if the pulsing stops at every tap.

3. Valves and the shower handset. A partly closed shut-off valve anywhere in the run, or a shower head with a shut-off on the handset that is only part open, gives the pump nothing to push into.

4. The hose at the pump. Check the flexible hose at the inlet and outlet for kinks. The short run from the pump to the hard line is where a hose folds when the pump has been disturbed. A replacement pump fitting with a straight run is often the fix.

5. No accumulator. An accumulator tank gives the pump somewhere to push against and smooths flow. Many RVs have none. Adding a small one on the pump outlet reduces cycling substantially and is the answer if every tap pulses and nothing above is restricted.

6. The pressure switch. Some pumps have an adjustable switch; a small adjustment widens the gap between cut-in and cut-out. Consult the pump's manual before turning anything.

If it started after a freeze

Assume a cracked fitting, faucet cartridge or pump housing. Ice splits things along seams that then weep under pressure, and a hairline crack in the pump body is easier to feel as a damp patch than to see.

When the pump itself is done

A pump that runs warm, moves little water and pulses at every tap after the checks above has a tired diaphragm. The 12 V water pump is the Black Series standard, and the replacement takes about an hour with the guide on the blog. If the camper has a tankless water heater, fix the pulsing first: those heaters need steady minimum flow to ignite, and a cycling pump delivers it in bursts.

Pumps, fittings and filters ship from Upland, California.